Does injecting CO2 harm fish?

There is a widespread concern that the pH swings from CO2 injection may hurt livestock.

The main reason why pH stability is important in planted aquariums is that pH levels usually reflect alkalinity (KH levels/carbonate hardness). The two parameters are connected - a change in KH will always results in a change in pH. However, the reverse is not true, pH can change significantly without changing the underlying KH much. (this happens during CO2 injection for example).

Alkalinity (KH) changes affects the osmoregulation of fish/animals. Sensitive shrimp, for example, are affected by sudden changes in carbonate salt concentrations in the water and dislike KH fluctuations of more than 3 degrees in a short period of time. So KH stability is important in a tank. In the days before testing for KH was regularly done, pH was used as a proxy for alkalinity - many tanks "worked" when the pH was stable as this indicated that the KH was stable.

However, if pH is changed without a change in KH, as in the case of CO2 injection, the changes in pH do not have the same impact. In CO2 injected tanks, the pH can change across the day as CO2 builds/dissipates, but the KH can remain stable.

Excessive CO2 levels and extreme pH levels (where acidity itself becomes an issue) will still affect the animals, but these are separate issues from pH fluctuations caused by CO2 per se.

When we do large 60-80% water changes in our CO2 planted tanks, the pH changes a full 1.0 unit in 20-30 minutes or less, yet we never lose sensitive shrimp or fish if the other parameters are kept constant. pH swings (1 full point+) also occur very frequently in nature, as CO2 levels build up overnight due to decomposition, but are quickly depleted during the light hours. 

Fish from acidic peat bogs will do well in a tank where the pH drops from 7 to 5.8 during CO2 injection, as the low point of the cycle pH (5.8) is within the range of their natural living conditions. However, alkaline water fish may not tolerate the dip into the pH 5.8 range well. It is not the fluctuation of pH, but the pH itself that is outside the natural range of the fish. Alkaline water fish similarly can survive a comparable pH swing from 8.5 to 7. It is important to distinguish the effects of a pH change from the effects of an extreme pH value in itself.

Click here to learn more about important water parameters for livestock.